This application is based upon and claims the benefit of Japanese Patent Applications No. 2000-269079 filed on Sep. 5, 2000, No. 2001-4028 filed on Jan. 11, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method for manufacturing a semiconductor pressure sensor having a closed cavity formed in a semiconductor substrate as a reference pressure chamber.
2. Description of the Related Art
Methods for manufacturing this kind of semiconductor pressure sensors are proposed by, for example, JP-A-8-236788, JP-A-6-18345, and JP-A-11-298009. These methods are as follows.
First, a cavity (recess) is formed on a surface of a semiconductor substrate made of silicon or the like. After that, another substrate made of semiconductor or the like is bonded to the surface of the semiconductor substrate to close the cavity. Then,a heat treatment is performed to the substrates. As a result, the two substrates are securely bonded, and a reference pressure chamber is provided with a predetermined internal pressure.
Further, a diaphragm for detecting a-pressure is formed from the semiconductor substrate at a portion corresponding to the reference pressure chamber, i.e., corresponding to a bottom portion of the cavity. After that, elements such as strain gauges and an integrated circuit a reformed in the semiconductor substrate by treatments such as diffusion and implantation. As a result, a semiconductor pressure sensor is completed.
According to studies and experiments by the inventors, however, it is revealed that the conventional manufacturing method has problems as follows.
Specifically, in the conventional method, after the closed cavity, i.e., the reference pressure chamber is formed in the semiconductor substrate, the elements such as the strain gauges and the integrated circuit are formed in the semiconductor substrate. In the steps following the formation of the reference pressure chamber, the semiconductor substrate is heated for performing ion-implantation, diffusion, and the like. If temperatures of such heat treatments are too high, the internal pressure of the reference pressure chamber is increased to deform the diaphragm.
The deformation of the diaphragm is accompanied by crystal defects thereof that are produced where stress concentrates on. For example, crystal defects are liable to be produced especially at an edge portion of the diaphragm. In the portion having crystal defects, electrical leakage and mechanical leakage easily occur to hamper the element formation. Further, the diaphragm may be damaged, and pressure may leak from the reference pressure chamber.
The present invention has been made in view of the above problems. An object of the present invention is to prevent crystal defects from being produced in a diaphragm of a semiconductor pressure sensor having a reference pressure chamber in a semiconductor substrate.
According to the present invention, in a method for manufacturing a semiconductor pressure sensor, a reference pressure chamber is formed by closing a cavity formed in a semiconductor substrate, and a diaphragm is formed from a part of the semiconductor substrate corresponding to the reference pressure chamber. A first heat treatment is then performed to the semiconductor substrate, at a first temperature T1 that satisfies a relation of:
T1 less than xe2x88x92430P0+1430
where P0 is an internal pressure (atm) of the reference pressure chamber at a room temperature. Accordingly, crystal defects can be prevented from being produced in the diaphragm. The internal pressure of the reference pressure chamber can be controlled by temperature and pressure for forming the reference pressure chamber, which are determined using the Boyle-Charles""s law.
The first heat treatment may be performed to form strain gauges, an insulation film, an integrated circuit or the like on the semiconductor substrate. Preferably, the reference pressure chamber is formed by performing a second heat treatment at a second temperature T2 that satisfies a relation of:
xe2x80x83T2 less than xe2x88x92430P0+1430
where P0 is the internal pressure of the reference pressure chamber at the room temperature. Accordingly, even when the diaphragm is formed simultaneously with the reference pressure chamber, crystal defects can be prevented from being produced in the thin diaphragm securely.